Typically, in the field of oil and gas exploration and recovery, analysis of seismic data obtained through seismic surveys can provide crucial physical parameters of subterranean rock formations. Generally, in seismic surveys, a seismic energy source is used to generate a seismic signal that propagates into the earth and is at least partially reflected by subsurface formations having different acoustic impedances. The reflections are recorded by seismic detectors located at or near the surface of the earth, in a body of water, or at known depths in boreholes. Conventional surface seismic surveys record compressional, or P-waves. Multicomponent seismic surveys record both P-waves, or compressional waves, and shear, or S-waves. Accordingly, the recorded seismic data is a combination of source wavelet and earth properties. As such, the goal of seismic data processing is to interpret earth properties to remove or minimize the effects of the source wavelet.
Generally, for P-waves, the vibrations occur in the direction of propagation of the waves, and for S-waves, the vibrations occur in a direction generally orthogonal to the direction of propagation of the waves. If the earth is isotropic with respect to the horizontal direction of wave motion, then a single S-wave arrival may be expected for each reflecting interface. As is often the case, however, subterranean formations behave anisotropically with respect to the horizontal direction for various reason, such as the geological layers contain fractures. Consequently, in an horizontally stratified azimuthally anisotropic media, two separate S-waves from each reflecting interface are recorded, where the S-waves arrive at different times, having propagated with different velocities.
Seismic data processing methods include azimuthal velocity correction and amplitude versus offset (AVO) analysis and inversion, amplitude versus offset and azimuth (AVOA or AVAZ—Amplitude Versus Angle and aZimuth) analysis and inversion of conventional three dimensional (3D) seismic data, and birefringence analysis of multicomponent 3D seismic data. The reflectivities, such as AVO, characterize subterranean properties and can serve as input to other inversion algorithms. In addition, the source wavelet is useful for a variety of seismic processing algorithms, including forward modeling of seismic data. The analyzed seismic data can provide useful information regarding the characteristics and parameters of the subterranean formation. Further, seismic detection of subsurface fractures has important applications in the study of unconventional rock formations such as shale plays, tight gas sands and coal bed methane, as well as carbonates, where the subterranean formations are naturally fractured reservoirs.
Information concerning these characteristics and parameters are often essential in a variety of fields such as underground transportation systems, foundations of major structures, cavities for storage of liquids, gases or solids, and in prediction of earthquakes. In oil and gas exploration, the information is important for determining optimal locations and orientations of vertical, inclined or deviated, and horizontal wells, minimizing wellbore instability, and formation break-out. Also, these characteristics are crucial to optimize the operating parameters of a commonly utilized technique for stimulating the production of hydrocarbons by applying hydraulic pressure on the formation from the wellbore. The outputs from the inversions of the embodiments of the present disclosure contain estimates of the elastic stiffnesses (velocities and anisotropic parameters) that can be used to predict lithology, porosity and the fluid content of the subsurface, as well as in predictions of intensity and orientation of fractures in subterranean formations.
The prior art seismic data processing methods have significant drawbacks and limitations. For instance, conventional Azimuthal AVO inversions provide only band-limited fractional elastic parameters estimates with reduced resolution and quality. That is, the bandwidth of seismic data is always less than the bandwidth of the desired reflectivities. Generally, conventional Azimuth AVO inversions involve the equation set forth by Rüger, A., 2002, “Reflection coefficients and azimuthal AVO Analysis in anisotropic media,” SEG geophysical monograph series number 10: Soc. Expl. Geophys (hereinafter “Rüger”). Rüger provides the equation showing the amplitude R with azimuth φ for narrow angles of incidence θ for an isotropic half-space over an HTI anisotropic half-space. The Rüger equation is nonlinear and can be linearized to simplify the inversion. Once in linear form, the parameters may be estimated in a straightforward fashion through least squares inversion. Transforming the parameters back to their original form results in estimates of the P-impedance reflectivity, isotropic gradient, anisotropic gradient and the isotropy plane azimuth of the HTI media. It can be shown that if fractures behave similar to the penny shaped crack theory of Hudson, disclosed in J. A., 1981, “Wave speeds and attenuation of elastic waves in material containing cracks,” Geophys. J. Royal Astronom. Soc. 64, 133-150 (hereinafter “Hudson”), then there is a direct relation between the anisotropic gradient and the fracture density. Further the isotropy plane describes the strike of the fracture. There are, however, a number of limitations to this method. In particular, the Rüger equation is derived for the case of an isotropic over HTI anisotropic half-space. It is not theoretically valid for the case of two anisotropic half-spaces. Further, there is a 90 degree ambiguity associated with the estimate of the isotropy plane, as indicated by Rüger. Moreover, for noise stability reasons, this method is usually implemented using a near offset approximation, which leads to bias in the parameter estimates.
Another known method is simultaneous prestack elastic inversion, such as that disclosed by Coulon, J-P, Lafet, Y., Deschizeaux, B., Doyen P. M. and Duboz, P., 2006, “Stratigraphic elastic inversion for seismic lithology discrimination in a turbiditic reservoir,” SEG Expanded Abstracts, 25, no. 1, 2092-2096 (hereinafter “Coulon”). This method addresses the band-limited fractional parameters by estimating isotropic elastic parameters. There are, however, still drawbacks to this method because it assumes an isotropic earth, thereby losing all the information associated with the anisotropic parameters such as fracture information. Further, overlooking these anisotropic parameters introduce systematic errors into such method.
In view of the drawbacks of methods known in the art, there is a great need for reliable and accurate estimation and modeling of elastic attributes of subterranean formations. The present disclosure provides for improved methods and systems that produce reliable estimates of elastic parameters of subterranean formations.